Active studies have been made to ionic conductors which are solid materials used as electrolytes in electrochemical devices, such as gas sensors and fuel cells. Major solid ionic conductors which have been known to date can be classified according to their crystal structures as shown in Table 1 [V. V. Kharton, F. M. B. Marques, A, Atkinson, Solid State Ionics, 174 (2004) 135-149. P. Lacorre, F. Goutenoire, O. Bohnke, R. Retoux, Y. Laligant, Nature, 404 (2000) 856-858. X. Turrillas, A. P. Sellars, B. C. H. Steele, Solid State Ionics, 28-30 (1988) 465-469].
TABLE 1ExamplesCrystal system & unit cell dimensionsStructural typecompound(Å)Fluoritedoped Zirconia: YxZr1-xO2-δ(YSZ)F-cubic: a ≈ 5.2doped Ceria: Ce0.9Sm0.1O2-δF-cubic: a ≈ 5.4Ce0.3Gd0.2O2-δ(CGO)δ-Bi2O3-based phases: Bi1.6Er0.4O3F-cubic: a ≈ 5.5PerovskiteLaGaO3-based phases: La0.5Sr0.2Ga0.75Mg0.2O3-δP-cubic: a ≈ 3.9(LSGM)LnBO3-based phases: La0.9Sr0.1AlO3-δP-cubic: a ≈ 3.8(Brownmillerite-type) Ba2In2O5-based phases:P-cubic: a ≈ 4.2BaIn0-Zr0.3O3-δ(disordered perovskite)(Cryolite) A3BO6-δ:Sr6Ta2O11cF-cubic: a ≈ 8.3Aurivillius series:γ- Bi2VO5.5-based phases (BIME VOX):I-tetragonal: a ≈ 3.9, c ≈ 15.5Bi2An−1BnO3n+δ(n = 1)Bi2V0.9Cu0.1O5.5-6PyrochloreA2B2O7:Gd2Zr2O7F-cubic: a ≈ 5.3ApatiteA10-x(MO4)6O2-5:La9.33Si6O26P-trigonal: a ≈ 9.7, c ≈ 7.2β-SnWO4La2Mo2O9-based phases: La2Mo2O9bP-cubic: a ≈ 7.2
All the materials listed in Table 1 have potentials. However, each may be advantageous or disadvantageous in certain applications because the above materials exhibit different ionic conductivities and physicochemical properties at various temperatures due to their structural characteristics such as crystal structures and ionic defect structures.
For example, in solid oxide fuel cell (“SOFC”) applications, it has been known that yttrium stabilized zirconia (“YSZ”) is the most suitable material for use as a high-temperature SOFC electrolyte. However, a doped ceria-type is more suitable for a low-temperature (lower than 600° C.) SOFC. In a high-temperature SOFC using any other electrolyte (doped ceria or La0.8Sr0.2GaO3-δ) than YSZ, materials such as La0.9Sr0.1AlO3-δ or Gd2Zr2O7 can be used as a protective layer of a cathode. An ionic conductor membrane for use in an oxygen pump should have both electrical conductivity and ionic conductivity. Accordingly, doped ceria, rather than YSZ with very low electrical conductivity, is suitable to be used in an oxygen pump. In addition, compounds having a cryolite structure or LaYO3-type high-temperature oxygen ionic conductors function as proton conductors at a wet atmosphere and a low temperature.
Since various properties are required according to the applications, it is very important to develop new-type materials having ionic conductivity. If a new material with a new crystal structure is developed, thousands of derivatives can be synthesized and prepared from the new material, which results in the rapid development of relevant technologies.